This project is concerned with the regulation of metabolism in preimplantation mouse embryos. The overall objective is to define the mechanism by which the mother can render delayed implanting embryos dormant, maintain them in that condition for several days, and then cause them to resume development, and implant. Experiments to be conducted in the upcoming grant period will focus on the regulation and function of a serieso f proteins that are synthesized and secreted by the embryo in the immediate preimplantation period. Synthesis of some of these proteins appears to be regulated in response to the signals that terminate the embryonic diapause associated with delayed implantation; others appear to be constitutive. Dormant embryos will be incubated with (3H) methionine and those undergoing reactivation will be incubated with (35S) methionine; the proteins that are synthesized and secreted will be mixed together and analyzed by 2-D electrophoresis on polyacrylamide gels under denaturing conditions. The proteins will be localized on the gels by fluorography or autoradiography and the ratio of (3H) to (35S) in the spots of interest will be determined with a scintillation counter. This ratio will be normalized to the (3H)/(35S) for overall protein synthesis in the same embryos to provide an estimate of the relative change in synthesis of each protein of interest. In this way it is hoped to determine whether synthesis of these proteins is regulated differentially. In addition, the temporal relationship of the appearance of these embryonic proteins to implantation-associated changes in the endometrium will be determined. Finally, experiments will be conducted to evaluate the possibility that the proteins secreted by mouse blastocysts interact with the endometrium by binding to the plasma membrane and thus may act as an embryonic signaling factor as has been shown to be the case for the protein (oTP-1) that is secreted by sheep blastocysts.